1. Field of the Invention
This invention relates to the formation of mesophase pitches useful for the production of carbonized fibers. More particularly, this invention relates to melt spinnable mesophase pitches that are suited to the production of pitch fibers.
2. Description of the Prior Art
Petroleum pitches with suitable softening points can be used satisfactorily as an impregnation material for electrodes, anodes, and carbon-carbon composites, e.g., carbon-carbon fiber composites, such as aircraft brakes and rocket engine nozzles. These pitches can also be used in the nuclear industry for the preparation of fuel sticks for a graphite moderated reactor. Furthermore, such pitches can be used in the production of carbon fiber precursors and carbonized fibers, i.e., carbon fibers and graphite fibers.
High strength per weight ratio of carbon and graphite fibers, alone or in composites, makes such fibers useful in sporting equipment, automobile parts, light-weight aircraft, and several aerospace applications.
U.S. Pat. Nos. 4,497,789 and 4,671,864, of Sawran et al., discloses producing substantially non-mesophasic pitch with a wiped-film evaporator.
U.S. Pat. Nos. 3,974,264 and 4,026,788, of McHenry disclose producing carbon fibers from pitch. A non-thixotropic, spinnable mesophase pitch having a mesophase content in the range of about 40 wt % to about 90 wt % is produced with shorter processing time by passing an inert gas through the pitch at a temperature in the range of 350.degree. C. to 450.degree. C.
U.S. Pat. Nos. 3,976,729 and 4,017,327, of Lewis, et al., disclose preparation of a non-thixotropic mesophase pitch while agitating the pitch during formation of the mesophase in order to produce a homogeneous emulsion of the immiscible mesophase and non-mesophase portions of the pitch. Improved rheological and spinning characteristics result from heating the pitch in an inert atmosphere at a temperature in the range of 380.degree. C. to 440.degree. C. for a time sufficient to produce a mesophase content in the range of 50 wt % to 65 wt % while agitating the pitch during the formation of the mesophase. A smaller differential between the average molecular weights of the mesophase and non-mesophase portions of the pitch also occurs.
U.S. Pat. No. 3,995,014 of Lewis discloses subjecting pitch to a reduced pressure during formation of the mesophase in order to substantially reduce the time otherwise required for its preparation.
U.S. Pat. No. 4,005,183 of Singer discloses a process for forming high-modulus, high-strength carbon fibers having a highly oriented structure containing crystallites. A mesophase-containing fiber is heated in an oxygen-containing atmosphere at 250.degree. C. to 400.degree. C. for a time sufficient to render it infusible, and then in an inert atmosphere to at least 1,000.degree. C.
U.S. Pat. No. 4,080,283 of Noguchi, et al., discloses continuous production of pitch from a heavy hydrocarbon oil by mixing with an inactive gas, such as nitrogen or steam, and heating at a temperature between 350.degree. C. to 500.degree. C. serially in a plurality of reactors with a portion of the liquid output from at least one of the reactors being recirculated. The liquid output of the final reactor can be introduced into an after-treatment duct-shaped chamber with an inactive atmosphere sealed therewithin to cool said liquid output. Such operation provides uniformity of reaction conditions in the reactor system.
U.S. Pat. No. 4,184,942 of Angier, et al. discloses producing an optically anisotropic, deformable pitch from a carbonaceous isotropic pitch by initially heating at between 350.degree. to 450.degree. C. and then extracting with an organic solvent system. The solvent-insoluble fraction is convertable into an optically anisotropic pitch.
U.S. Pat. No. 4,208,267 of Diefendorf, et al. discloses producing an optically anisotropic, deformable pitch from the solvent-insoluble fraction of a carbonaceous isotropic pitch that has been extracted with an organic solvent, e.g. such as benzene or toluene. The solvent-insoluble fraction is heated for 10 minutes or less to temperatures between 230.degree. C. to 400.degree. C., to yield an optically anisotropic phase of greater than 75%. The phase contains less than about 25 wt % of substances unextractable with quinoline at 75.degree. C.
U.S. Pat. No. 4,209,500, of Chwastiak discloses producing: both a single-phase, essentially 100% anisotropic mesophase pitch having number average molecular weight below 1000, a net pyridine insoluble content no greater than 60% by weight, a softening temperature no greater than 350.degree. C., and a viscosity no greater than 200 poises at 380.degree. C. and carbonaceous fibers therefrom. An inert gas is passed at a sufficient rate through an isotropic carbonaceous pitch while heating the pitch at between 380.degree. C. to about 430.degree. C. to agitate sufficiently to produce a homogeneous emulsion of the mesophase and to ensure removal of volatile low-molecular weight components. By "inert gas" is meant to be a gas which does not cause a significant change in the chemical nature of the pitch materials being contacted at the process conditions of temperature and pressure.
U.S. Pat. No. 4,402,928, of Lewis, et al. discloses producing a carbon fiber from precursor material such as ethylene tars, ethylene tar distillates, gas oils derived from petroleum refining, gas oils derived from petroleum coking, aromatic hydrocarbons, and coal tar distillates having at least 50% by weight which boils under about 300.degree. C. and at least about 70% by weight which boils under 360.degree. C. One of these precursor materials is heated in batches under pressure to obtain a pitch which is solvent extracted to obtain an 70% by weight or more mesophase portion. The insoluble mesophase portion can be converted into a carbon fiber.
U.S. Pat. No. 4,460,557 of Takashima et al., discloses producing carbon fibers by heating a starting pitch between 340.degree. C. to 450.degree. C. under a stream of inert gas, such as nitrogen, at up to atmospheric pressure, melt spinning the resulting material to form pitch fibers, infusibilizing, and then carbonizing or graphitizing.
U.S. Pat. No. 4,504,455, of Otani, et al., and European Patent application No. 813058930, Publication No. 0054437, of Otani each discloses a carbonaceous pitch comprising quinoline soluble dormant anisotropic hydrocarbon components that are partially hydrogenated mesophase portions of a mesophase pitch. The carbonaceous pitch is optically isotropic in nature with a dormant mesophase orientable when subjected to shear forces. The dormant mesophase pitch is prepared by hydrogenerating the mesophase of a mesophase pitch until substantially all the mesophase is quinoline soluble. Production of a carbon fiber from these pitches is also disclosed. In the European application, the dormant mesophase pitch is prepared by solvent extracting mesophase pitch into quinoline insolubles and quinoline solubles and then hydrotreating the quinoline insoluble portion. The higher the measured quinoline insoluble fraction, the higher tends to be the amount of mesophase components that are present.
U.S. Pat. No. 4,528,087, of Shibatani, et al., discloses producing, with for example extraction, a mesophase pitch containing 40% or more of quinoline solubles by heating a pitch having an aromatic hydrogen content of 50% to 90% at a temperature in the range of 430.degree. C. to 550.degree. C. while passing an inert gas thereover until at least 40% mesophase is formed.
U.S. Pat. No. 4,529,498, of Watanabe discloses producing a 100% mesophase pitch of quinoline-insoluble and quinoline-soluble components, by (1) heating to a temperature of 360.degree. C. to 450.degree. C. a petroleum derived pitch while stirring under a low molecular weight hydrocarbon gas atmosphere at atmospheric or super-atmospheric pressure until the mesophase content is 10% to 50%, to form a heat treated pitch, (2) holding without stirring the heat treated pitch at a temperature in excess of 280.degree. C., but below 350.degree. C., to permit separation into a layer of non-mesophase and a layer of mesophase, and (3) separating the non-mesophase layer from the mesophase layer. High-strength, high-modulus carbon fibers can be produced from the resulting mesophase layer.
U.S. Pat. No. 4,529,499, of Watanabe adds to the method of U.S. Pat. No. 4,529,498 by subjecting separated non-mesophase material to steps (1), (2), and (3) at least 3 times to prepare a 100% mesophase composed only of quinoline-insoluble and quinoline-soluble components.
U.S. Pat. No. 4,575,411, of Uemura, et al., discloses producing a melt-spinnable carbon fiber precursor pitch with a softening point between 200.degree. C. to 280.degree. C. by heating a film of 5 mm or less of a carbonaceous pitch at a temperature of 250.degree. to 390.degree. C. and at a pressure of 100 mm mercury or less until the precursor pitch, contains 40% or more mesophase material. The mesophase pitch has 0 wt % to 15 wt % of an anisotropic quinoline-insoluble phase and 85 wt % to 100 wt % of an anisotropic quinoline-soluble phase.
Chwastiak's method of U.S. Pat. No. 4,209,500 involving stripping requires a relatively long time to obtain mesophase spinnable pitch from a base pitch. Not only is stripping time consuming, but also high-molecular weight materials can be carried over with low-molecular weight materials during stripping due to foaming and the like. However, even without foaming, the volatile carry over from stripping includes potentially useful components hard to recover due to the presence of highly diluting stripping gases and high cracked materials that increase with residence time at high temperatures.
The method of Diefendorf, et al., in U.S. Pat. No. 4,208,267 involves a solvent extraction to remove low-molecular weight component which is rather difficult to practice.
Carbonaceous materials (sometimes called fiber precursors) for the manufacture of carbon or high-strength graphite fibers, conventionally employ polyacrylonitrile or mesophase pitch. However, preparation of mesophase pitch requires a time consuming and expensive batch process of heating at an elevated temperature for a number of hours, as shown by Lewis, et al., in U.S. Pat. No. 3,967,729, by Singer, in U.S. Pat. No. 4,005,183, and by Schulz, in U.S. Pat. No. 4,014,725. Improper heating can increase viscosity of mesophase pitch too much, rendering it unsuitable for spinning. Also, polyacrylonitrile is often a more expensive feedstock than is mesophase pitch.
This invention involves a method for producing very good yields of reliably uniform melt spinnable mesophase pitch.